...What if the "exchange scheme" was simpler, and a single one for *all* types of media content? (e.g.: book, song, movie, record, ...)
Something like: you choose what you want, and pay for it; you get the first half of it and have a longish time to check it out (say, a month); within such "exchange window", you can substitute the work you downloaded with (the first half) of another work of the same category, even multiple times; when you have found something that you like, you confirm the purchase and get the whole file; if the window expires, the work you are currently previewing is automatically confirmed.